The Distant Sun

Earth reaches aphelion on the 4th of July. Curiously,
our planet is warmest when we are farthest from the Sun.

July
4, 2003: You learned it in school. Astronomers say it all
the time. It's the Truth: "Earth circles the Sun."

Well... almost.

Earth does travel around the Sun, but the path is not a perfect
circle. It's an ellipse, slightly lopsided. One end is a bit
closer to the Sun than the other.

On July 4th, 2003, our planet will reach the distant end --
a point astronomers call "aphelion." We'll be farther
from the Sun than we are at any other time of the year.

Above: The sun setting behind the Statue of Liberty
is a little more distant than usual on the 4th of July.

"All planets in our solar system travel around the Sun
in elliptical orbits. It's Kepler's 1st Law," says University
of Florida astronomy professor George Lebo. "The eccentricity
of Earth's orbit is 1.7%. In January when we're closest to the
Sun (perihelion), the distance is 147.5 million km. This weekend
we will be 152.6 million km away--a five million kilometer difference."

Editor's Note: Do you have trouble
remembering the difference between perihelion and aphelion? An
old astronomer's trick is to recall that the words "away"
and "aphelion" both begin with the letter "A".

A distant sun means less sunlight for our planet. "Averaged
over the globe, sunlight falling on Earth at aphelion is about
7% less intense than it is at perihelion," says Roy Spencer
of NASA's Global Hydrology and Climate Center (GHCC).

Then why is it so warm outside?

"Seasonal weather patterns are shaped primarily by the
23.5 degree tilt of our planet's spin axis, not by aphelion or
perihelion," continues Lebo. "During northern summer
the north pole is tilted toward the Sun. The Sun climbs high
in the sky, and days are long. That's what makes July
so hot." (Note: seasons are reversed in the two hemispheres,
north and south. So July is generally cold in the southern hemisphere.)

But there's more to the story: Says Spencer, "the average
temperature of the whole earth at aphelion is about 4oF
or 2.3oC higher than it is at perihelion." (See
the global
temperature data at the GHCC web site.) Our planet is actually
warmer when we're farther from the Sun. Strange but true.

Above: Earth's land-masses are found more north of the
equator than south. But it wasn't always that way. Image credit
and copyright: the PALEOMAP
Project.

This happens because continents and oceans aren't distributed
evenly around the globe. There's more land in the northern hemisphere
and more water in the south. During the month of July the land-crowded
northern half of our planet is tilted toward the Sun. "Earth's
temperature (averaged over both hemispheres) is slightly higher
in July because the Sun is shining down on all that land, which
heats up rather easily," says Spencer.

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Physicists would say that continents have low heat capacity.
"Consider the desert," says Bill Patzert, an oceanographer
at NASA's Jet Propulsion Laboratory. "At night the desert
is cold, perhaps only 60o F (16o C). When
the Sun rises in the morning the temperature might jump to 100o
F (38o C) or more." Such mercurial behavior is
characteristic of materials like rocks and soil with low heat
capacity. It doesn't take much sunlight to substantially elevate
their temperature.

Water is different. It has high heat capacity. "Let's
say you went sailing off Malibu Beach at noon," continues
Patzert. "The offshore temperature might be 75o
F (24o C) -- pretty pleasant!" What happens after
sunset? "The temperature drops, but only a few degrees because
the heat capacity of the ocean is so high."

All this explains why July is our planet's warmest month:
Northern continents baked by the aphelion Sun elevate the average
temperature of the entire globe. January, on the other hand,
is the coolest month because that's when our planet presents
its water-dominated hemisphere to the Sun. "We're closer
to the Sun in January," says Spencer, "but the extra
sunlight gets spread throughout the oceans." Southern summer
in January (perihelion) is therefore cooler than northern summer
in July (aphelion).

Right:
Earth's orbit is eccentric but not nearly so much as the orbits
of Mars or Mercury. In this diagram solid lines trace each planet's
elliptical path around the Sun. The dotted lines show circular
orbits with the same mean radius. For more information, please
visit Bridgewater College's Interactive
Planetary Orbits web site.

"Another notable difference between summers in the two
hemispheres is their duration," adds Lebo. According to
Kepler's 2nd Law, planets move more slowly at aphelion than they
do at perihelion. As a result, Northern summer on Earth is 2
to 3 days longer than southern summer -- which gives the Sun
even more time to bake the northern continents.

If you're feeling baked during the 4th of July holiday weekend
and wish that aphelion brought more relief, there is something
you can do: Take a hint from the watery southern hemisphere.
Locate the nearest swimming pool and dive in; feel the water's
high heat capacity. A little physics can be refreshing
....